In modern telecommunication systems where it is required that documentation, checking or charging operations be carried out automatically, the need arises to detect data which are expressed by the presence or absence of direct currents on particular circuits and to supply such data to a load such as, for instance, an electronic computer.
For a proper evaluation of these data it is first of all necessary that incoming direct currents, lacking mutual electrical correlation in most instances, be converted by the detecting device into outgoing signals related to a given reference voltage without regard to the electric state of the various circuits to be examined.
Further, these outgoing signals must be at a level compatible with that required for the operation of logic circuits of computing system.
Finally, it is indispensable that in the event of failure no detecting device should affect the normal operation of a telecommunication network.
To solve those problems, use has been made of detecting devices that employ, as sensors for the presence or absence of current, magnetic cores with rectangular hysteresis loops traversed by conductors which carry the signal currents to be detected (writing windings) and are connected to units designed to perform interrogation operations and to restore the initial state of such cores, i.e. to zeroize or reset them, upon each interrogation.
As compared with other systems for criteria detection, those based on the use of cores offer the guarantee of a low sensitivity to electric disturbances present on the circuits to be examined and a high electric decoupling between reading and writing circuits, thus obviating any interference, even in the event of failure, with the normal operation of the controlled network. Additionally, the use of cores makes it possible to modify in a simple way the threshold value of the current to be sensed.
Further, it has now become the practice, in particular where criteria have to be detected from a large number of circuits, to arrange such cores in groups in the form of matrices on common supports and to read them by means of time-sharing techniques. An arrangement of this kind allows great simplification of the circuitry of the detecting device.
Core matrices known in the art are provided with duplicated logic devices for reading and writing operations and include self-diagnostic units which employ groups of cores not connected to the circuits from which criteria are derived, i.e. cores without writing windings.
In these matrices, however, duplication has been accomplished in such a way that only one of the two unit groups of the duplicated device will be operating, whereas the other is constantly kept as a standby and will intervene only in the event of failure of the first group.
This entails the severe drawback of rendering an on-line checking of standby devices impossible, and as a consequence these devices may be unable to accomplish their task when required to intervene because of a failure of the main chain. In this situation all data collected by the detecting device obviously get lost and this may result not only in a hindrance to the normal management of a telecommunication network but also in serious economic losses for the operating administration when such data relate to accounting operations.
Another shortcoming of this sort of device is inherent in the procedure of checking the operation of the chain of devices being used; to state whether troubles have to be ascribed to interrogation devices or reading devices, use must be made of several horizontal and/or vertical rows of cores without reading windings.
It is therefore obvious that the detection of malfunctioning devices can be performed only to the detriment of the data-collection capacity of detecting devices, whose number must consequently be increased, which in turn gives rise to space problems in such areas as telephone exchanges where there is already a high density of equipment.
A further disadvantage of conventional core matrices is due to the fact that there are no tools offering a protection against failures which, occurring in only one or two components of duplicated testing units, make is automatically impossible for the other components of that unit to operate, as when for instance a scanning circuit goes on sending interrogation current to a row or column of the matrix; in this situation, a switchover to the standby unit would cause several rows or columns to be simultaneously interrogated and therefore the detected signal samples would not correspond to the actual situation.